<template>
    <div class="list">
       <ul>
           <li class="item" 
               v-for="item of letters" 
               :key="item"
               :ref="item"
               @click="handleLetterClick"
               @touchstart.prevent="handleTouchStart"
               @touchmove="handleTouchMove"
               @touchend="handleTouchEnd"
            >
                {{ item }}</li>
       </ul>
    </div>
    
</template>

<script>

export default {
    name: 'CityAlphabet',
    props: {
        cities: Object
    },
    computed: {
        letters () {
            const letters = []
            for (let i in this.cities) {
                letters.push(i)
            }
            return letters
        }
    },
    data () {
        return {
            touchStatus: false,
            startY: 0,
            timer: null
        }
    },
    updated () {
        this.startY = this.$refs['A'][0].offsetTop
    },
    methods: {
        handleLetterClick (e) {
            this.$emit('change', e.target.innerText)
        },
        handleTouchStart () {
            this.touchStatus = true
        },
        handleTouchMove (e) {
            if (this.touchStatus) {
                if (this.timer) {
                    clearInterval(this.timer)
                }
                this.timer = setTimeout(() => {
                    const touchY = e.touches[0].clientY - 79
                    const index = Math.floor((touchY - this.startY) / 20)
                    if (index >= 0 && index < this.letters.length) {
                        this.$emit('change', this.letters[index])
                    }
                }, 16);
                
                
            }
        },
        handleTouchEnd () {
            this.touchStatus = false
        }
    }

}
</script>

<style lang="stylus" scoped>
@import '~styles/variables.styl'
    .list
        position absolute
        top 1.58rem
        right 0
        bottom 0
        width .4rem
        display flex
        flex-direction column
        justify-content center
        .item
            line-height .4rem
            text-align center
            color $bgColor
</style>